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Date: Tue, 24 Dec 2002 15:33:40 +0100
From: Rene Tschaggelar
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Subject: Re: Making a fast scope probe
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Jim boy wrote:
> When I connect a 250MHz scope probe on X10 to the inbuilt 1ns rise time 1Mhz
> square wave in my oscilloscope with the ground lead floating, there is
> ringing on the edges. This is down to the inductance of the loop formed by
> the inner conductor of the probe and the ground of the scope resonating with
> the input capacitance of the scope. Connecting the ground of the probe to
> the scope ground via it's BNC eliminates post of the ringing. The loop is
> much smaller since the outer conductor of the probe now transfers most of
> the ground signal.
> On the advice of a link that was posted here a few months back on how to
> construct a "wide-band" probe to 1Ghz consisting of a 100R resistor on the
> inner conductor of co-ax at the sensing end terminated in in it's
> characteristic impedance I noted the following:-
> 1. With the outer conductor only connected at the BNC, there should be
> ringing but there wasn't. The rise time was poor
> 2. Connecting the outer conductor at the sensing end to the BNC slightly
> improved the rise time
> 3. Removing the 50R characteristic impedance made the rise time
> significantly worse with still no ringing seen.
> My conclusion is that without the terminating resistance, you've merely got
> a bog-standard x1 probe that is limited by it's ability to charge the input
> capacitance of the scope. The terminating impedance improves things only
> because it dominates the input capaciance at the expense of less signal at
> the input. Reducing it to 10R improves the rise time further.
> So what is a better solution? How about splitting the terminating impedance
> as 49R in series with 1R across the input capacitance? I'll try this after
> the new year in the hope of seeing ringing.
There are active probes that have rather good specs. Even though their
allowable voltage range is somewhat limited, they are a good buy.
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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